WO3 thin films were synthesized on FTO (fluorine doped SnO2 on glass) by the hydrothermal method at 180 degrees C using different deposition times and subsequently annealed at 350 degrees C and 550 degrees C in air. The results of XRD and Raman showed complete phase transformation from WO3 hydrate into gamma-monoclinic WO3 after annealing at 550 degrees C. SEM analysis showed significant effects of deposition time and annealing on the surface morphology of the WO3 thin films. WO3 films were examined by the Scotch tape test and showed superior adhesion for samples synthetized for 2 h compared with samples synthesized for 15 h. The photoelectrochemical properties of the WO3 samples were measured. The highest photocurrent density of similar to 1.9 mA/cm(2) (E = 1.4 V vs. Ag/AgCl, 369 nm LED, irradiance 100 W/m(2)), IPCE = 0.69 at 369 nm, and 2 mA/cm(2) under simulated solar irradiation (AM1.5) was achieved for a layer thickness of 3.5 mu m.

In contrast to the industrial-scale production of H2O2, electrochemical or photoelectrochemical synthesis is an environmentally friendly alternative. In this study, the photoelectrochemical generation of H2O2 was investigated using a hematite (alpha-Fe2O3/FTO/glass) photoanode in combination with a gas diffusion electrode (GDE) modified by the incorporation of tin(II) phthalocyanine (SnPc) in its hydrophilic layer. The experiments were conducted in a photoelectrochemical cell with two compartments separated by a proton exchange membrane, under an applied bias and AM1.5 irradiation (100 mW/cm2). The amount of H2O2 generated was quantified through chemical analysis using visible light spectrophotometry of the electrolyte. To assess the process efficiency, the Faradaic efficiency (FE) was calculated. The optimal configuration employed air as the inlet gas for the GDE and phosphate buffer (pH 6.4) as the electrolyte in the cathodic compartment. The combination of the hematite photoanode and the GDE modified with SnPc was the most effective for H2O2 photoelectrochemical generation. The highest FE values achieved were 52.4 % for the GDE (O2 reduction to H2O2) and 0.4 % for the hematite photoanode (H2O oxidation to H2O2).

The competitive photoelectrochemical reactions bromide vs. water (photo)oxidation in an acidic medium (pH = 1.7) were studied on four semiconducting oxides viz. hematite (α-Fe2O3), pseudobrookite (Fe2TiO5), tungsten trioxide (WO3) and titanium dioxide (TiO2). The experiments were carried out with thin film electrodes on conductive substrates (FTO/glass) in a vigorously stirred batch reactor under AM1.5 irradiation. Electrical bias was applied to the electrodes, exclusively promoting valence band processes leading to oxidative pathways. The initial Faradaic efficiency (FE0) for bromine generation was determined by chemical analysis (UV-Vis absorbance) of the electrolytes.The semiconducting materials used as photoanodes showed a similar ability to oxidize bromide to bromine. For 0.02M sodium bromide, a Faradaic efficiency of 100% was obtained for α-Fe2O3, WO3 and TiO2, while the highest value for pseudobrookite was 90%.From further analysis, as to competition kinetics, the ratio of the rate constants of water oxidation and bromide oxidation, vH2O / kox, was estimated. α-Fe2O3 was the most successful compound among the studied semiconductors.

This study describes the photoelectrochemical degradation of model aromatic pollutants, viz. a herbicide ("monuron", 3-(4-chlorophenyl)-1,1-dimethylurea) and an antimicrobial preservative (benzoic acid) on WO3 films prepared by aerosol pyrolysis. Crystalline WO3 films had a monoclinic structure, 4 mu m thickness and a photocurrent density of 1 mA cm-2 at 1.2 V vs. RHE and illumination under 1 sun (simulated). The chemical and photoelectrochemical stability of WO3 films at various pH was evaluated. The dissolution rate at pH 1 and 4 was low (0.5 nm center dot h- 1), whereas at pH 6 an increase in the dissolution rate to 15 nm center dot h- 1 was observed. The course of the photoelectrochemical oxidation of selected pollutants was examined using HPLC. The achieved Faradaic efficiency of photoelectrooxidation of 1 mM benzoic acid divided by the number of charge equivalents per 1 mol of oxidised compound R, f/z, was 13% and 12% at pH 1 and 4, respectively, and for 1 mM monuron 7.2% at pH 1 and 7.5 % at pH 4. Although the concentration of pollutants decreased significantly, UV-VIS and TOC (total organic carbon) analyses showed the prevalent presence of intermediates during the degradation. During the degradation of benzoic acid, hydroxybenzoic acid (salicylic acid) was found by HPLC as an early intermediary product.

Semiconducting Fe2O3 thin films were deposited on SnO2:F (FTO) and Pt substrates by reactive high-power impulse magnetron sputtering combined with electron cyclotron wave resonance plasma (HiPIMS + ECWR). Fe2O3 films were fabricated either by sputtering from a single Fe target or by co-sputtering from an additional Sn target. Plasma parameters during co-sputtering were measured by an RF probe system enabling the comparison between HiPIMS + ECWR and only HiPIMS conditions used for the film deposition. As deposited Fe2O3 films were post-annealed in air at 450 degrees C and 650 degrees C, respectively. It was shown that as deposited Fe2O3 films were amorphous but became crystalline after post-annealing at 450 degrees C. Further increase of annealing temperature to 650 degrees C did not improve significantly the crystalline structure of the Fe2O3 film. All post-annealed Fe2O3 films exhibited photocurrents in the anodic region; generally, hematite films annealed at 650 degrees C exhibited higher photocurrents than those annealed at 450 degrees C. Films doped by Sn co-sputtering had higher photocurrents than films only doped by Sn diffusion during post-annealing from the FTO substrate. Hematite films on Pt substrate doped by Sn co-sputtering and post-annealed at 650 degrees C exhibited the highest photocurrents. It was verified by XPS analysis with ion sputtering depth profiling that Pt atoms also diffuse from the Pt substrate into the Fe2O3 film during the post-annealing at 650 degrees C and can, similar to Sn in the case of an FTO substrate, act as a dopant.

In this work, the photoresponse of Ti doped hematite electrodes was optimized by adjusting the thickness and the level of doping. The electrodes were then covered by a TiO2 overlayer by dip coating (DC) or by atomic layer deposition (ALD) to improve chemical stability. Coverage of hematite by a dip-coated sol-gel TiO2 film of thickness up to 135 nm resulted only in a small decrease in photocurrent (by about 30 %). A similar decrease in photocurrent (about 25 %) was observed after the coverage by a very thin (2 nm) ALD TiO2 film while the increase of the thickness to 8 nm led to almost complete suppression of photocurrent. This behaviour can be explained by the different morphology and structure of the overlayers. Although the surface morphology of sol-gel TiO2 films was very smooth and dense, penetration of such films by the electrolyte occurred even for a thickness of 135 nm. On the other hand, much thinner ALD TiO2 films (8 nm) exhibited almost no electrolyte penetration. Such different blocking properties of the two TiO2 overlayers are in direct agreement with the observed dissolution rate in acidic media - the better the blocking properties of an overlayer the better the chemical stability of stratified hematite/titania photoelectrode.

Pseudobrookite, Fe2TiO5, precursor films were obtained by spray pyrolysis (SP) at 550 degrees C on fused silica and FTO (F-doped tin oxide on borosilicate glass) using iron (III) acetylacetonate (FeAcAc) and titanium diisopropoxide bis(acetylacetonate) (DIPTiAcAc) in methanol. SP was followed by annealing in air from 600 degrees C to 1000 degrees C for various durations, and for T >= 750 degrees C, phase pure pseudobrookite was obtained. (Photo)electrochemical experiments of Fe2TiO5 electrodes in junctions with aqueous electrolytes showed n-type behaviour of the material with a maximum photocurrent of 0.35 mA/cm(2) under simulated AM1.5 sunlight. A valence band energy between 6.6 and 6.8 eV was estimated using the electrochemical results. The position of the Fe2TiO5 valence band enables the passage of (photogenerated) holes in hematite into a pseudobrookite layer and further on towards an electrolyte, if a hematite/pseudobrookite stratified film would be applied in a solid / liquid junction. The valence band potential is not positive enough for producing OH center dot radicals, only solvent oxidation and reactions which do not require OH center dot radicals can proceed.

The photoelectrochemical degradation of selected aromatic substances, acid orange 7 (AO7), salicylic acid (SA), benzoic acid (BA), and 4-chlorophenol (4-CP) was studied on hematite (α-Fe2O3) and compared with titanium dioxide (TiO2), both deposited as thin films on conducting substrates (FTO/glass). Batch type reactors were used under backside and front side illumination. Electrical bias was applied on the semiconducting electrodes, such that only valence band processes leading to oxidative pathways were followed. The initial Faradaic efficiency, f0, of degradation processes was determined from the UV–Vis absorbance decrease of the starting materials. f0 for 1 mM AO7 degradation in 0.01 M sulphuric acid was found to be 7.5

Iron (III) oxide, in the form of hematite (α-Fe2O3), is a n-type semiconductor which is photoactive in the visible spectral region. Therefore, use in photoelectrocatalysis and photoassisted water electrolysis may be suggested. For such implementations, stability of contacts with liquid phases is mandatory. Hematite is stable in alkaline media but less stable in acidic media. For the first time the coverage of porous photoactive Sn doped hematite by thin capping layers of TiO2, deposited by Atomic Layer Deposition (ALD) and its impact on photocurrent and chemical stability of hematite is shown. The nominal thicknesses of the TiO2 ALD coatings were 0.5, 2 and 7.5 nm. The presence of the TiO2 coatings was evidenced by X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy (HR-TEM) and scanning TEM coupled with energy dispersive X-ray (EDX) spectroscopy. HR-TEM analyses revealed that the TiO2 capping layers were amorphous and conformal. Exposure of uncovered hematite layers to 1 M sulfuric acid led to a nominal dissolution rate of 0.23 nm/h which was halved when a TiO2 ALD coating (7.5 nm thin) was applied. Due to mismatch of the valence band positions of the two semiconductors, photocurrents were strongly diminished as the capping layer thickness was increased. Post-calcination of as deposited ALD films on hematite resulted in an increase of photocurrent, which only exceeded photocurrents of pristine hematite when the ALD thickness was not more than 0.5 nm.

The possibility of protection of Fe2O3 (hematite) against photocorrosion in aqueous electrolytes by thin layers of TiO2 and SnO2 deposited by ALD (atomic layer deposition), was investigated. Sn-doped hematite layers, as obtained in this study by aerosol pyrolysis had significant roughness and porosity. ALD is very successful in applying conformal films to such structures. The nominal coverage by ALD films was varied between 0.5 and 7.5 nm. The presence of the TiO2 and SnO2 films was evidenced by XPS. Photocurrents were strongly diminished as the capping layer thickness was increased. The Faradaic efficiency, f, of photocorrosion in acidic media (0.01 M H2SO4) was decreased from 0.026 to 0.014 and to 0.010 by capping with either a 2 nm thick overlayer of TiO2 or of SnO2, respectively. The latter had also a positive influence on the long term photocurrent stability.

Sn-doped hematite (Fe2O3) films were prepared by aerosol pyrolysis (AP) on fluorine doped tin oxide (FTO), titanium and stainless steel. Photoactive electrodes were obtained in all cases and the photosensitivity had an onset around 650 nm. Maximum incident photon to electron conversion efficiency (IPCE) was 0.3 at 300 nm for samples deposited on FTO. The Faradaic efficiency of the photocorrosion reaction was found to be 0.47% for an unprotected FTO/hematite electrode in H2SO4. The Faradaic efficiency of this dissolution reaction decreased to 0.3% for a hematite electrode covered with a 65 nm thick dip coated layer of TiO2, and to 0.17% for a sample with a spray coated TiO2 layer, thus proving the beneficial role of TiO2 in protecting hematite against photocorrosion.